This invention relates to LED luminaries and more specifically, to a control system for providing white light with selectable color temperature and dimming level.
Within the past few years LED technology has advanced remarkably to a point where the efficiency of light generated by an LED array matches or even exceeds the efficiency of incandescent lamps. In many lighting applications, Red, Green and Blue LED arrays are employed to generate a conventional white light. By properly mixing the lumen generated by each group of the Red, Green and Blue LED it is possible to control the xe2x80x9ccolor temperaturexe2x80x9d of the white light generated by the LED array. Theoretically, color temperature of a light source is defined as the temperature of a plankian radiator (ideal light source) whose radiation has the same chromaticity as that of the light source, and is measured in Kelvins. To an ordinary observer the color temperature refers to the color of the white light. A cooler white lightxe2x80x94similar to the light generated by commercial fluorescent lampsxe2x80x94has a lower temperature, whereas a warmer white lightxe2x80x94similar to the light generated by residential incandescent lampsxe2x80x94has a higher temperature.
The term chromaticity is applied to identify the color of the light source regardless of its lighting level or lumen. When the chromaticity of different light sources is equal, the color of the light from each light source appears the same to the eye regardless of the lighting level. The chromaticity of a light source is represented by chromaticity coordinates. An example of such coordinates is the CIE 1931 chromaticity diagram, in which the color of the emitted light is represented by x, and y coordinates.
Practically, the color temperature of an LED array is defined as the correlated color temperature. The term correlated color temperature refers to a light source whose chromaticity coordinates are not exactly equal to any of the chromaticity coordinates of an ideal light source. The correlated color temperature of a real light source, such as a lamp, is thus defined as the temperature of an ideal light source whose perceived color most closely resembles that of the real light source at the same brightness and under specified viewing conditions. In this context, the present description employs the terms color temperature and correlated color temperature interchangeably.
The correlated color temperature and the dimming level of an RGB LED array depend among other things, on the operating temperature of an LED, the age of the LED and batch-to-batch variations in production of the LED.
Thus, there is a need for a control mechanism for white LED luminaries that can maintain a specified light level for all desired operating conditions.
In accordance with one embodiment of the present invention a white luminary LED is made of three types of LED light sources, using a plurality of Red, Green and Blue LEDs. A light control system is configured to maintain the color temperature and the lumen output level of the emitted white light. The control system comprises a feed-forward temperature compensation arrangement and an optical feedback control system to maintain the target white light. The junction temperature and the light output of the LEDs are sensed and are fed into the light control system.
The temperature feed-forward compensation arrangement is employed to correct the deviation in the target color temperature and the color-rendering index of the white light. A processing means, such as a feed forward temperature compensator means is configured to provide required lumen output fractions of the Red, Green and Blue LED light sources, in response to the junction temperature of the LEDs and the target white light. The required lumen outputs from the Red, Green, and Blue LED light sources for a target white light are calculated by using the chromaticity coordinates of the target white light and the chromaticity coordinates of the light emitted by the LED light sources based on the junction temperature.
In accordance with one embodiment of the invention the chromaticity coordinates for the light emitted by the Red, Green and Blue LED light sources are computed as a function of junction temperature in advance and stored in a memory means. In accordance with another embodiment of the invention the required lumen output fractions of the Red, Green, and Blue LED light sources can also be computed off-line as a function of junction temperature and stored in the memory means.
A lumen output module in combination with a lumen output controller are configured to maintain the light output generated from the LED light sources equal to the light output value provided by the feedforward temperature compensator, regardless of junction temperature, aging and batch-to-batch variation.